Introduction
The leaf economics spectrum (LES), capturing a suite of key traits provides a useful framework for identifying plant investment strategies (Wright et al. 2004). Species characterised by high specific leaf area (SLA) and nutrient concentration, but low tissue density and lignin concentration, tend to exhibit faster resource absorption (an acquisitive strategy), while species at the opposite end of the LES have a more conservative strategy (Wright et al. 2004; Reich 2014). Plants make a cost-benefit trade-off between root absorption and leaf resorption in nutrient economy (Norby & Jackson 2000; Kou et al. 2017). The acquisitive-strategy species running in a ‘fast lane’ may invest more in root-nutrient absorption but less in leaf-nutrient resorption, while conservative-strategy species running in a ‘slow lane’ may present opposite patterns. This active economic trade-off in nutrient acquisition pathways could cause a subsequent passive trade-off between resorption and decomposition, two primary processes after leaf senescence and fall (Killingbeck 1996; Cornwell et al. 2008). Therefore, a process-based ‘Get (absorption)-Save (resorption)-Return (decomposition)’ continuum (hereafter, the ‘GSR’ continuum; Fig. 1) may emerge. Although trait-based LES is recognised, its linkages with these processes have been less or separately explored (Freschet et al. 2010, 2012; Riva et al. 2019; Rosenfiled et al. 2020). Whether the ‘GSR’ continuum exists and conforms to the ‘fast-slow’ leaf economics spectrum has not been tested empirically.
The emergence of the ‘GSR’ continuum may be associated with soil nutrient status and plants’ carbon (C) investment strategy for nutrient acquisition. In nutrient-poor habitats, acquisitive-strategy species invest more in roots to forage for nutrients and less in leaf nutrient resorption (Hodge 2004; Kramer-Walter & Laughlin 2017; Zhao et al. 2020), leading to nutrient-rich leaf litter and faster decomposition. In contrast, the conservative-strategy species may allocate more effort to leaf nutrient resorption and invest less in the belowground (Deng et al. 2018), leading to nutrient-poor leaf litter and slower decomposition. However, the continuum might not emerge in nutrient-rich habitats where plants acquire nutrients at a relatively low cost. This discontinuity is more likely to occur in the active trade-off between root nutrient absorption and leaf nutrient resorption (Kou et al. 2017), because high nutrient availability may not impel plants to make such an economic trade-off. The legacy effects of resorption on decomposition, i.e. the passive trade-off, however, could persist regardless of soil nutrient availability (Deng et al. 2018), since the nutrient concentration in senesced leaves (resorption proficiency) largely determines the decomposability of leaf litter. Despite this theoretical assumption, our knowledge of the linkage among these processes for plants spanning from acquisitive to conservative strategies remains limited.
The absorptive roots of woody plants are symbiotic with mycorrhizal fungi, which may influence the continuity of these processes. Ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) associations are among the most common mycorrhizal types (Brundrett & Tedersoo 2017), but differ in such processes as nutrient absorption (Chen et al.2016), nutrient resorption (Zhang et al. 2018), and litter decomposition (Keller et al. 2019; Jiang et al. 2021). Ectomycorrhizal plants utilise organic nutrients directly, as ECM fungi facilitate organic nutrient uptake through secreting extracellular enzymes (Talbot et al. 2008; Lin et al.2017). However, AM plants mainly scavenge inorganic nutrients mineralised by free-living microbes (Read & Perez-Moreno 2003; Smith & Smith 2011). Moreover, ECM plants have more conservative nutrient cycles and lower leaf nutrient concentrations than AM plants (Chapman et al. 2006; Phillips et al. 2013). Considering the negative relationships between leaf nutrient resorption and green leaf nutrient concentrations (Killingbeck 1996; Kobe et al. 2005; Vergutz et al. 2012), ECM plants may have higher nutrient resorption and slower litter decomposition than AM plants (Keller et al. 2019; Xu et al.2020). Given the distinct modes of nutrient-cycling processes between ECM and AM plants (Lin et al. 2017), unearthing the mycorrhizal controls over the ‘GSR’ continuum will facilitate predictive and mechanistic understanding of the whole-plant nutrient economy.
Here, we present the first empirical evidence of a direct link among these nutrient-associated processes, i.e. absorption, resorption, and decomposition and their relationship with LES in 15 co-occurring subtropical tree species. We determined the associated stoichiometric parameters of four nutrient pools (soils, absorptive roots, green leaves, and senesced leaves) of these species to assess the root absorption and leaf resorption of nitrogen (N) and phosphorus (P). In addition, we simultaneously measured SLA, leaf tissue density (LTD), and lignin concentration in green leaves as well as five morphological and architectural absorptive-root traits: root diameter (RD), specific root length (SRL), root tissue density (RTD), average root length (ARL), and branching intensity (BI) that are closely associated with nutrient absorption. We then performed a microcosm experiment to quantify the leaf-litter decomposition rate. Our previous studies had reported that nutrient foraging strategies (Kou et al. 2018a, b; Li et al. 2019) and litter decomposition (Jiang et al. 2018, 2019) of plants are more driven by P than N in subtropical forests, indicating that P could be the most limiting nutrient for these processes. Our overarching hypothesis is that the ‘GSR’ continuum would exist and run on the P economy and conform to the ‘fast-slow’ LES among these tree species. Based on the mycorrhizal-associated nutrient economy framework (Phillips et al. 2013), the contrasting nutrient economy modes of AM (inorganic-nutrient economy) and ECM (organic-nutrient economy) species can cause divergences in absorption, resorption, and decomposition. Thus, we further hypothesised that the continuum would differ markedly between AM and ECM species.